The present invention relates generally to the finishing of paper and paperboard, and more particularly to an improvement in a conventional waterbox calender for finishing paperboard to provide enhanced stiffness with a minimal loss of caliper or bulk.
It is common practice in the paper industry to add moisture to a paper web at the machine calender during the papermaking process. Although other methods of moisture application have been proposed, it is most commonly carried out by waterbox calendering. A typical example of a conventional waterbox calender is disclosed for instance in U.S. Pat. No. 2,130,530 to Fletcher. However, an inherent problem with the use of a waterbox calender is a lack of control of the amount of liquid picked up by the web in the calender nip. During conventional waterbox calendering, the amount of liquid supplied to the calender nip is such that the nip is flooded. Thus the quantity of liquid picked up by the web is determined by the calender roll diameter, operating speed, calender nip pressure, and web characteristics (thickness, sizing level, and roughness). Accordingly the amount of moisture applied to a web using a waterbox calender cannot be controlled independently of the operating characteristics of the calender which are selected to achieve the desired paper or paperboard properties. For example, a typical papermachine is used to make a range of paper thicknesses or basis weights. However, if the operating characteristics of the waterbox calender are selected to give the desired paper properties for a specific basis weight, those operating characteristics may not be appropriate for other basis weight products. This is a decided disadvantage, since it is known that the stiffness of a paper web is determined by the densification of the fibers during calendering, which is related to the moisture content of the web at the time of calendering. In this regard, it is also known that increasing the density of a substantially homogeneous structure such as paper or paperboard during calendering, particularly with waterbox calendering, will, in general decrease stiffness. On the other hand, stiffness may be enhanced where the densification is controlled, as for example, with composite structures, where the outer layers can be made dense to have a higher elastic moduli than the center layer to achieve the so called I-beam effect. Thus it would be desirable to provide for a typical papermachine a more versatile method for adding moisture to the web prior to machine calendering to achieve an I-beam effect, than is presently possible using a conventional waterbox.
Other methods for adding moisture to a paper web include the application of steam or the use of water sprays. Application of steam to a web to increase its moisture content is possible, but it requires that the web be cooled for efficient condensation of the steam. Moreover, in addition to the equipment required for cooling the web, it is difficult to condense the quantity of steam required to impart the same smoothness that can be achieved by waterbox calendering. Spraying liquid directly onto a web is another method for increasing the moisture content of the web. However, conventional spraying systems which apply moisture directly to the web lack the uniformity required to produce a smooth surface. In addition, the lack of cross direction uniformity, and the production of wet streaks caused by the overlap of sprays from adjacent nozzles, results in nonuniform smoothness and caliper profiles. A third method for adding moisture to a web is to apply the liquid directly to the web using an offset roll or the like. An example of this method is shown in U.S. Pat. No. 4,973,441, wherein an apparatus is disclosed for offsetting a liquid or a plasticizer directly to a web before calendering to achieve a compressibility gradient in the web. However, this method like the use of steam, requires the addition of equipment to the papermachine prior to the machine calender, and in practice such offsetting devices have been found to be unsatisfactory for achieving a uniform cross direction moisture profile in the web.
Thus while the use of a waterbox calender is generally agreed to be the preferred method for adding moisture to a paper web, the problems inherent with conventional waterbox calendering have yet to be solved. Furthermore, the other conventional methods known for the application of moisture to webs have thus far been inefficient. Nevertheless, it would be advantageous to create within a substantially homogeneous structure such as paper or paperboard, the same structural characteristic (i.e., I-beam effect), that may be achieved in a composite structure with outer densified layers and a bulky interior. The objective, then, is to apply a minimum amount of moisture uniformly across the web so that upon calendering, only the outer layers of the web are densified (like a composite structure), to achieve enhanced stiffness. This result is accomplished with the practice of the present invention wherein the means for applying moisture to the web is of a novel type, specifically, a brush spray device capable of generating a mist of moisture, particularly as disclosed in U.S. Pat. No. 5,314,119, the volume of which can be controlled independently of the operating characteristics of the calender. Accordingly, the novelty of the present invention lies in the replacement of the waterboxes typically used on a machine calender with the brush spray devices more fully described hereinafter to achieve the desired results.